Magnetic induction accelerator



Feb. 6, 1951 Filed Aug. I 5, 1947 K- J. R. WILKINSON ET AL MAGNETICINDUCTION ACCELERATOR Fig.1.

2 Sheets-Sheet 1 The]? Attorn ey.

Patented Feb. 6, 1951 MAGNETIC INDUCTION ACCELERATOR Kenneth J. R.Wilkinson and Christopher J. Milner, Rugby, England, assignors toGeneral Electric Company, a corporation of New York Application August5, 1947, Serial No. 766,360 In Great Britain October 4, 1945 Section 1,Public Law 690, August 8, 1946 Patent expires October 4, 1965 4 Claims.(01.250-27) This invention relates to apparatus for accelerating chargedparticles, such as electrons, by means of magnetic induction efiects.

It is an object of the invention to provde a new and improvedaccelerator of the aforesaid ype.

It is a further and more specific object to provide improved means fordiverting orbitally moving electrons after their acceleration hasreached the desired degree.

In the drawings, Fig. 1 is a cross-section view of a magnetic fieldproducing device constructed in accordance with the invention; Fig. 2 isa plan view of the device with clamping members omitted; Fig. 3 showsthe method of connecting the concentric windings in parallel; Fig. 4shows the manner in which the auxiliary transformer is connected to theparallel concentric w ndings in order to provide impulsive change in theflux density for the purpose of orbit expansion or contraction; and Fig.5 is a plan view of the device showing the manner in which the clampingmembers are applied. Equipments for producing electron streams of highvelocity by induction acceleration have become Well-known. They involvea magnetic field-producing device providing a central flux path in whicha time varying flux of high amplitude is present, the flux density beingreatest at the center or axis of the field and diminishing towards theperiphery, an evacuated discharge tube of circular or annular'form beingpresent in the flux path and substantially linked by the flux. Anelectron stream is produced in the evacuated tube and the high rate ofchange of flux linking the tube causes the electrons produced to rotatearound the axis of the field while the electrons are prevented frompermanently leaving their orbits until they have attained their desiredvelocity by the field existing at or near their orbit.

In order to balance the centrifugal force of the electrons the fluxdensity near the orbit must be accurately controlled with relation bothto the flux which links the orbit and to the radius of the orbit.

- As a result of the high intensities of magnetic field required inpractice and the presence of an air gap at the center of the flux path,a source of considerable kva. may be needed to energize the magneticfield-producing device. However, with the construction employed in ourinvention, the magnetic field-producing device is capable of providingthe required field intensities and flux densities having appropriaterelated values with a considerable reduction in the required electricalkva. input.

A magnetic field-producing device for the purpose set forth' accordingto the invention provides for a'substantial portion of the time-varyingflux which must link the electron orbit for the purpose of acceleratingthe electrons tangentially, to pass substantially or wholly throughoutits length through a magnetic core, while the magnetic flux densitywhich is to occupy the nonmagnetic evacuated orbit space for the purposeof counteracting the centrifugal force on the electrons is restricted toa space of minimum volume.

The magnetic field-producing device preferably comprises a magnetic coreconstruction having a central core which is excited by two concentricwindings in such a manner that, when an A. C. voltage is applied to thewindings, flux linking both windings is substantially in time phase withthe leakage flux not linking both windings, while the leakage flux notlinking both windings is made to traverse the non-magnetic annularvolume occupied by the circular electron orbit space, whereas all otherflux paths comprise highly permeable magnetic material substantiallythroughout their lengths. The relative turns in each winding are sochosen in relation to the reluctance of the electron orbit space fluxthat the flux o linking the equilibrium orbit bears substantially therelation:

In this relation, while the electrons are accelerated Bo is the fluxdensity at, and R0 is the radius of, the equilibrium orbit.

By this means we fulfill the equilibrium condition necessary foraccelerating the electrons by magnetic induction. The further conditionthat the equilibrium orbit shall be stable is that where B is the fluxdensity at radius R in the neighborhood of the orbit radius. Thiscondition is achieved by. appropriate shaping of the orbit flux spacepoles coupled with the disposition of the two windings.

The two concentric windings may be connected in series or parallel. Whenthe windings are connected in parallel, as is to be preferred, the twowindings are wound in the same sense and embody differing numbers ofturns, the outer winding having fewer turns than the inner wind-'- ing.If, however, the windings are connected in series, they are wound inopposite sense and the inner winding has a fewer number of turns thanthe outer winding.

When the process of electron acceleration is complete, or sufficientlyadvanced, it may be necessary to expand or contract the electron orbitso as to produce collision with a suitable target, and in order toprovide for orbit expansion or contraction, the flux linking theequilibrium orbit is made to change impulsively, relative to the fluxdensity of leakage flux not linking both windings at the orbit. For thecase where two concentric windings are connected in parallel, expansionor contraction may conveniently be effected by applying an impulsivevoltage in each cycle of the applied voltage at the instant at whichelectron energy is substantially a maximum, by means of an auxiliarytransformer having two secondary windings with turns respectivelyproportional to, and connected in series with, the parallel concentricwindings.

In order that the invention-together with further objects and advantagesthereof, may be more readily understood and carried into effect we willnow describe, with reference to the accompanying drawings, an embodimentof the same.

Referring to Fig. l, the magnetic field-reproducing device embodying theinvention consists of an inner annular core I, an outer annular core 2,and an intermediate annular core 5 in two parts. These several cores arelinked by radial laminated members 3 and 4. The cores I, 2 and 5 may bein the form of spir-ally wound strips of magnetic material, although theinner and outer cores may be of any other suitable laminatedconstruction. Thus it is to be understood that in place of spirallywound cores We might employ standard core limbs both for the inner andouter core paths. The device is energized by an inner winding 6 and anouter winding 1 concentric therewith, the outer winding being, forconvenience in affording access to the orbit space, in two sections, asindicated, which may be connected either in series or in parallel. Theevacuated tube 8 in which the orbitally moving electrons are containedis introduced into the air gap between the two parts of the core 5.

In Fig. 5 we have shown in plan the method which may be adopted forsecuring together the parts of the magnetic field-producing deviceillustrated in Figs. 1 and 2. It is necessary to split circumferentiallythecore members I, 2, 5 to prevent them from forming short-circuitpaths, and to adopt a similar precaution with regard to clamping ringsemployed in the construction. The splits in the ring 2 are indicated at13 in Fig. 2. In Fig 5 the two halves of the semi-circular cores areheld together at their outer periphery by clamping half-rings I4 ofL-shape in crosssection which are bolted to similar half-rings at thebottom of the device by bolts l5. At the center half-rings I6 and atransversely located clamping plate I 1 hold the inner periphery of thedevice, a central bolt l8 extending through the hollow central portionof the core construction. Pairs of plates l9 bear at their edges on theadjacent bridging members and serve to hold the core parts 5 inposition, clamping bolts 20 passing through the spaces between 3 and 4to engage plates, similar to plates 19, located on the inner faces ofthe parts 5. These latter plates are of non-magnetic material, such asbronze.

The rincipal fluxes 1 and 2 are shown in the section of Fig. l; n beinglinked to both coil 6 and coil 1, ea being linked only to coil I. Thevoltage per turn of coil 6 is therefore proportional to 1 and thevoltage per turn of coil 1 is proportional to 1+2. The requirement that(in and 2 shall be in the same direction, is made by ensuring, in thecase of a parallel connection of windings 6 and I, that the turnscomprising the winding 1 shall be less in number than those comprisingthe parallel winding 6. For a series connection it would be necessarythat the turns comprising the winding 1 should be greater in number thanthose comprising the winding 6, but as we have previously mentioned theparallel arrangement is to be preferred because by this means the timephase angle between fluxes 1 and 2 can be made smaller.

The conditions to be met in this apparatus are well known and have beenhereinbefore specified by the two expressions given above. In apractical example, using a parallel arrangement of windings 6 and l, wemay cite, for example, that the winding 6 comprises 20 turns eachcarrying .800 amperes, and the windings I together comprise 14 turnseach carrying 1200 amperes, while the current to be supplied from asource of A. C. power is the difference between these two currents,namely 400 amps. Again, in this example, the voltage of supply was 2000and frequency 200 cycles.

The arrangement of the windings as shown in Fig. 3 is to be modified forthe purpose of orbit expansion, which demands a relative change inamplitude between 4n and qiz, by the use of the auxiliary seriestransformer arrangement shown in Fig. 4. In the preferred form of thistrans,- former an impulsive voltage and current are applied to a primarywinding II which excites by means of the common core l2 two secondarywindings 9 and In which are in series with the concentric windings 6 and1 respectively. Relative turns and polarity of the windings 9 and H) areso chosen that the resultant excitation by load currents in windings Band l of the core I2 is sensibly zero. This requirement is met byarranging that the turns 9 and ID are respectively proportional to theturns of 6 and I. When an impulsive voltage and current is applied towinding II at the time appropriate for shifting the orbit, theexctitation of core l2 assumes a finite magnitude whose algebraic signis determined by the polarity of the impulse with respect to thealternating current source of power. Since the impulse causes themagnitude of the excitation of core [2 to become other than zero, it isapparent from Fig. 4 and transformer theory that the pre-existingrelative values of the voltages across windings 9 and Ill, and hencewindings Sand 1, can no longer exist. Thus, the relationship between theamplitudes of l and 1+2 is modified; the relationship between the flux 0linking the electron orbit as previously stated and the flux density Beat the orbit is similarly altered; and the electron orbit shifts. Byappropriate choice of polarity of the impulse, the voltage of winding I0 due to the impulse is additive with the voltage from the A. C. powersource; consequently, the voltage of winding 9 is also additive andgreater than that of winding I0. Therefore, the ratio or the rates ofchange of i and 1+2 is increased; hence, the ratio of the rates ofchange of 1 and 2 is increased and the ratio of the rates of change ofetc and Bo will increase, thereby causing the orbit to expand.

It will be understood that the impulsive voltage can be provided by anysuitable impulsing circuit which is synchronized and properly phasedwith respect to the variations of main flux. Examples of such circuitsare given in United States Patent No. 2,394,072 issued February 5, 1946,to W. F. Westendorp and assigned to the General Electric Company.

From the aforegoing description it is obvious that various advantagesmay accrue when our invention is employed. One important advantagereadily apparent is that the electron orbit may be expanded orcontracted to cause the electrons to impinge upon a target withoutresorting to windings situated upon a portion of the magnetic circuitstructure of the accelerator. Thus, by the use of our invention theimpulsive disturbance of the flux relationship required to shift theorbit may be obtained from windings in the form of a transformer placedexteriorly of the accelerator, thereby permitting the more effectiveutilization of the air gap in which the evacuated tube is positioned.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. Apparatus for accelerating charged particles along a re-entrant pathcomprising a magnetic assembly having a structure which provides amagnetic circuit for flux linking said path and which further provides amagnetic circuit for flux traversing the locus of said path, a pair ofcoils for energizing said magnetic assembly respectively located insideand outside of the locus of said path, a source of time-varyingexcitation, circuits respectively connected between said source and saidcoils for concurrently exciting the coils to establish a desired balanceof said magnetic fluxes, and a source of impulsive Voltage for changingsimultaneously the energization of said coils so that the balance ofsaid fluxes is disturbed and the path of said particles abruptlyshifted.

2. Apparatus for accelerating charged particles along a re-entrant pathcomprising a magnetic assembly having a structure which provides amagnetic circuit for flux linking said path and which further provides amagnetic circuit for flux traversing the locus of said path, a pair ofcoils for energizing said magnetic assembly respectively located insideand outside of the locus of said path, a source of time varyingexcitation, circuits respectively connected between said source and saidcoils for concurrently exciting the coils to establish a balancedrelationship of 1 said magnetic fluxes such that the flux traversin thelocus of said path is suflicient to confine said charged particles tosaid path while the flux linking said path causes said charged particlesto accelerate, and a source of impulsive voltage for changingsimultaneously the energization of said coils to disturb the balancedrelationship of said magnetic fluxes and thus abruptly to shift the pathof said particles.

3. Apparatus for accelerating charged particles along a re-entrant pathcomprising a magnetic assembly having a structure which provides amagnetic circuit for flux linking said path and which further provides amagnetic circuit forand a source of impulsive voltage for disturbingsaid balanced relationship to shift the path of said particlescomprising a transformer having two secondary windings, each of whichsecondary windings is serially connected with one of said concentricwindings.

4. In an apparatus for accelerating charged particles along a re-entrantpath, the combination which includes a magnetic assembly havingconcentric inner and outer cores defining a magnetic flux circuit forflux which links said path and an annular core intermediate said innerand outer cores defining with said outer core a magnetic flux circuitfor flux which traverses the locus of said path, inner and outerconcentric windings for energizing said assembly disposed respectivelywithin the space between said inner and said intermediate cores and thespace between said intermediate and said outer cores, said windingshaving a relative turns ratio with respect to the reluctance of saidlatter magnetic flux circuit so as substantially to fulfil the relationwhere is the flux linking, Bo the flux density at, and R0 the radius of,the re-entrant path, a source of time-varying excitation, circuitsrespectively connected between said source and said windings to supplyexcitation to said Windings, and a source of impulsive voltage forchanging simultaneously the energization of said wind ings therebydisturbing said relation and abruptly shifting said path.

KENNETH J. R. WILKINSON.

CHRISTOPHER J. MILNER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,103,303 Steenbeck Dec. 28, 19372,331,788 Baldwin Oct. 12, 1943 2,394,072 Westendorp Feb. 5, 1946 OTHERREFERENCES Journal of Applied Physics, Nov. 1945, vol. 16, pages657-600; ibid., Jan. 1947, vol. 18, pages 8-11.

